Effect of amorphous carbon coating on the formation of solid electrolyte interphase and electrochemical properties of a graphite electrode

Coating graphite negative electrodes of lithium-ion batteries with amorphous carbon layer can significantly improve the battery performance. We investigated the effect of amorphous carbon coating on the formation of a solid electrolyte interphase (SEI) on the graphite surface by performing gas adsorption measurements, surface analysis, and electrochemical impedance measurements. The specific surface area of graphite particles uniformly coated with amorphous carbon is reduced by almost a factor of two, and the irreversible capacity at the first charge/discharge cycle significantly decreases. The SEI film consists of LiF in particulate and O-based coating uniformly distributed at the edges. Hence, the amorphous carbon coating increases the amounts of F and O atoms on the SEI surface and reduces capacitance C ' and the Faraday current at high temperatures. Although the C ' value decreases by approximately 80% after SEI formation, the graphite electrode with an amorphous carbon coating exhibits enhanced C' retention properties. Because the frequency and temperature dependences of the electrode capacitance are strongly affected by the amorphous carbon coating, an electric double layer is likely formed at the graphite/SEI interface. The difference in capacitive behavior can be attributed to the activity of Li insertion/desorption reaction and capacity fading during storage at elevated temperatures.